The field of the invention is that of heat exchangers, and more particularly, heat exchangers for use in domestic furnaces.
In one prior art form of a conventional domestic furnace, air to be heated (room air) is circulated around a serpentine heat exchanger for heat transfer to the conditioned room air. The heat exchanger defines a passageway for the flow of hot combustion gases conventionally produced by burning a fuel such as oil, gas, etc. The hot products of combustion pass through the heat exchanger thereby transferring heat to the conditioned room air, which is exhausted through a suitable flue.
To facilitate the heat transfer, heat exchangers preferably cause a turbulent flow within the fluid streams which exchange heat. Turbulent flow is achieved by superimposing an unsteady fluctuating velocity distribution on a steady mean flow pattern. By providing such a steady mean flow pattern, i.e. an average rate of flow, the furnace can reliably maintain air intake and exhaust. By superimposing an unsteady fluctuating velocity distribution, i.e. shifting subcurrents, on the steady flow pattern, the fluid stream transfers heat through the interface media of the walls of the heat exchanger. Providing a sufficiently turbulent flow assures that the fluid streams interact properly with the interface media for the efficient exchange of heat. However, turbulence also creates stress on the heat exchanger structure. Thus, it is desirable to provide a structurally secure heat exchanger which provides a sufficiently turbulent flow to assure proper functioning of the heat exchanger.
Heat exchangers are classified by the relative direction of the fluid streams which exchange heat. With aligned fluid flow channels, the streams run either parallel or counterflow. Streams with a parallel flow orientation are those which flow in relatively the same direction. Streams with a counterflow orientation travel in relatively opposite directions. With the fluid flow channels positioned relatively transversely, the streams flow with a cross flow orientation. Counterflow represents the most efficient method of transferring heat within a heat exchanger since it assures the greatest temperature differential between the heat exchanging fluid streams.
To most efficiently utilize a furnace, the heat transfer from the combustion products to the conditioned room air is maximized. A serpentine heat exchanger is conventionally used to continuously increase the temperature of the conditioned room air as it flows over the heat transfer surfaces of the heat exchanger. When disposed in a counterflow orientation, conventional heat exchangers maximize their heat transfer efficiency, although certain installations require a more uniform, albeit somewhat less efficient, distribution of heat transfer.
U.S. Pat. No. 4,739,746 (Tomlinson) describes a furnace having a serpentine heat exchanger for selectively providing either a parallel or counterflow heat transfer arrangement. Although the serpentine heat exchanger of the Tomlinson patent provides improved selective functioning, the increasing cost of fuel for furnaces creates a need for heat exchangers which have greater efficiencies in order to minimize heating costs. However, conventional designs do not effectively employ the full advantage of the heat transfer surfaces. Thus what is needed is a heat exchanger which fully utilizes the potential heat transfer surfaces of the heat exchanger.